There is a considerable need worldwide to generate more pure water for potable and industrial uses, which itself creates both domestic and industrial waste streams requiring further treatment prior to recycling or discharge to the environment. The use of cartridge filter elements that require frequent cleaning or changing, although an efficient means of filtration, creates a waste stream of used cartridge elements which is a further waste disposal problem.
It is also known to carry out downflow and upflow filtration of suspended solids from a liquid stream using a granular media bed, either atmospherically in concrete, plastic, metal or GRP tanks or above atmospheric pressure inside a suitably designed pressure vessel manufactured in metal, plastic or GRP. Many variants are known in distribution and collection methodology, in the media selection, and in chemical enhancement and filtration velocities.
The space and weight requirements of current filtration devices is also becoming a limiting factor in expanding or new build water treatment facilities, particularly for use on offshore oil exploration and production platforms. There is a need therefore for a compact media filter that minimises discharge to the environment, uses the minimum media wash water, and has the ability to operate successfully under the type of motions experienced on floating production facilities.
It is also known to use a radial media filter that has the ability to wash its media whilst online. This provides advantages, both in terms of the throughput versus space and weight, and allows the filter to operate whilst under the influence of gravitational forces due to motion.
Although the better designs go a good way to answering some of the problems associated with radial filtration and the backwash of its media, they do have some features which limit their effectiveness, such as a peripheral inlet screen that defines an annulus to receive water to be filtered, and distribute it radially into the media bed. In practice this screen is expected to retain media and inhibit it from entering the inlet distribution annulus. Should a very fine media be utilised then this screen's open area will be required to have a very small gap size to retain the fine media, and as such will act as an edge filter for large particles at the filter's inlet, so that filtration takes place on the outside of the inlet screen in the inlet annulus area.
This is undesirable as it will require a method of cleaning this screen or at least a strainer upstream of the filter to ensure that no particles larger than the screen gap size are delivered to the filter. Also, the central collection screen of this unit has no media support material, which means that the inner collection screen must also have a very fine screen gap size, one example of which would be a wedge wire screen.
The fluxrate (volumetric flowrate of raw water per unit area) for a given flowrate in a radial filter increases as the flow approaches the centre of the filter. The Reynolds number increases dramatically compared to its number at the outer diameter of the media bed. This can create large pressure drops and force fine suspended solids to migrate through the bed, particularly whilst the bed is in wash mode whilst on line.